Fix backend order split shipments action

When moving an item to a existing shipment from the same stock location
it would make two requests to remove items from the same shipment.
Instead it needs to remove from one shipment and then add the same
quantity to another shipment

[Fixes #3161]

I still would like to do more refactoring on the backend split shipments section but I just spent way more time than I thought I would on this fix. Hopefully I'll get back to it soon enough.

This should fix the random failures on the backend build over the ci.